Stock markets that trade elec0tronically (e.g., The Nasdaq Stock Market™), as opposed to those that trade on a trading floor (e.g., the New York Stock Exchange™), can conduct trading for extended periods of time.
As traditional, floor-traded, stock markets require interaction on a trading floor between human traders to accomplish stock trades, this trading most typically occurs during normal trading hours. However, electronically traded stock markets allow for automated trading between traders via computers before and after normal trading hours.